1. Field of the Invention
The present invention relates to a composite extruding apparatus of rubber including a screw extruder and a gear pump, and is suitable for use by connecting it to a die head for extrusion-molding of the rubber. Further, the present invention relates also to a method of extruding unvulcanized rubber with the use of the composite extruding apparatus.
2. Description of the Background Art
As a method of producing an extrusion-molded product of rubber such as a tire tread, a weather strip, a door seal, or a hose of an automobile, there are known a method in which a vulcanizable rubber composition obtained by kneading a rubber material with additives such as a reinforcing agent, a filler, a plasticizer, a vulcanizer, and a vulcanization promoter is fed to an extruder and extruded to a desired shape from the extruder via a die head, and vulcanized with a vulcanizing apparatus after being subjected to a lamination process or the like, such as in the case of a tire tread, or a method in which the vulcanizable rubber composition is guided to a vulcanizing apparatus for vulcanization after extrusion, and then is cut to a desired length to obtain a product, such as in the case of a weather strip.
Generally, a screw extruder is used for the above-mentioned extrusion. In this screw extruder, rubber is conveyed by a screw in a barrel, where the rubber is subjected to friction, shear, and compression between an inner surface of the barrel and an outer surface of the screw, whereby the rubber is self-heated and plasticized to have a viscosity that enables molding. In a conventional type extruder in which a die head is directly connected to the screw via a screen mesh, it has been considered that the longer the length L of the aforesaid screw is, the better it is, in view of the necessity of ensuring a predetermined molding pressure to the die head after the rubber passes through the screen mesh. In other words, it has been assumed that the screw must be provided with a feeding part, a compressing part, and a metering and discharging part, and hence the ratio L/D of the length L to the outer diameter D of the screw has been set to be within a range from 12 to 16 for a rubber at room temperature.
However, if the length of the screw is increased, it requires a larger motive force to that extent, and temperature control for heating and cooling will be required that respectively corresponds to the feeding part, the compressing part, and the metering and discharging part, thereby necessitating a complex temperature control mechanism. Moreover, even if the length of the screw is increased as described above, a plasticizing effect comparable to that of a Banbury mixer cannot be obtained. In order to improve the degree of plasticization with the use of the conventional type extruder, the speed of the screw rotation must be increased. This raises the temperature of the discharged rubber too much and generates scorching of the rubber, making the rubber unmoldable and decreasing the throughput. In addition, there is a problem that, by increasing the length of the screw, the material will remain in a larger amount in changing the lots.
On the other hand, it is known in the prior art that, by connecting a gear pump to a tip end of a screw extruder, the pressure of feeding rubber to a die head is maintained constant to enable precision molding (publication of Japanese Patent application No. 05-116200/1993(A1) (which has the same contents as U.S. Pat. No. 5,156,781)). This known art is constructed in such a manner that the metering and discharging part among the feeding part, the compressing part, and the metering and discharging part of the screw is transferred to the gear pump to allow the two functional areas of the feeding and heating part and the compressing part to remain in the screw, and that the pressure loss generated at the screen mesh and the breaker plate, which is a supporting body of the screen mesh, between the screw extruder and the gear pump is restrained to the minimum extent. Therefore, the total length of the screw has been such that the aforesaid ratio L/D is within a range from 4.5 to 8.5, and also the shape of the rubber passageways before and after the screen mesh and the breaker plate has been complex.
The present invention provides a composite extruding apparatus of rubber in which, as compared with the known composite type extruding apparatus using a gear pump in combination, the ratio L/D is further reduced to decrease the screw length while maintaining the same screw diameter, whereby the material residue is reduced. The present invention also provides a composite extruding apparatus of rubber in which the throughput can be increased while maintaining the same screw diameter, and the screw motive force is reduced. Further, the present invention provides a composite extruding apparatus of rubber in which the temperature control is facilitated to prevent rubber scorching, and extrusion molding of a vulcanizable rubber composition is made possible at a low temperature. The present invention provides a composite extruding apparatus in which the shape of the rubber passageways before and after the screen mesh can be simplified. In another aspect, the present invention provides a method of extruding an unvulcanized rubber with the use of the aforesaid extruding apparatus.
Thus, the present invention provides a composite extruding apparatus of rubber in which a gear pump is connected to a tip end of a screw extruder via a screen mesh having a breaker plate, and a die head is connected to an ejecting side of the gear pump, wherein the screw extruder plasticizes a fed rubber composition mainly by friction with an inner surface of a barrel and by heating from outside and extrudes the rubber composition from the screen mesh to a space on an inlet side of the gear pump; the screen mesh rectifies the rubber composition passing through the screen mesh towards the gear pump while stirring and plasticizing the rubber composition by friction and shear; and the gear pump further stirs and plasticizes the rubber composition, which is fed to the space on the inlet side, by shear and compression, and gives a forcing pressure to the die head.
In other words, the screw extruder used in the present invention needs to be substantially provided only with the feeding part among the feeding part, the compressing part, and the metering and discharging part, which are needed in conventional extruding apparatus, thereby eliminating the need for the compressing part and the metering and discharging part. Therefore, the length of the screw can be reduced to a great extent while maintaining the thickness (D), thereby all the more simplifying the temperature control mechanism.
The ratio L/D of the length L to the outer diameter D of the aforesaid screw is preferably set to be within a range from 1 to 4, particularly within a range from 2 to 3. Further, it is sufficient that the aforesaid screw extruder can plasticize a rubber composition by heating, and can feed the rubber composition to the gear pump by the rotation of the screw against the resistance of the screen mesh, so that the screw extruder does not need a forcing pressure needed for the rubber composition to pass through the die head. Therefore, the driving motor of the aforesaid screw is reduced in scale as compared with conventional ones. However, if the aforesaid ratio L/D is less than 1, the plasticization by heating the rubber composition will be insufficient, whereas if the ratio exceeds 4, the length will be too large to be economical.
Further, the screen mesh equipped with the breaker of the present invention rectifies the rubber composition, which passes through the screen mesh and its supporting breaker plate towards the gear pump, while stirring and plasticizing the rubber composition with friction and shear. Namely, the rubber composition ejected from the screw extruder and having a twisted shape is passed through the screen mesh and the holes of the breaker plate, whereby the aforesaid rubber composition is stirred and further plasticized by friction and shear at that time while being accompanied by a pressure loss. Therefore, it is sufficient that the rubber passageways before and after the aforesaid screen mesh has a straight tubular shape, and the bore diameter of the rubber passageways can be made equal to the diameter of the outlet of the screw extruder, thereby all the more simplifying the structure of the rubber passageways.
The gear pump connected to the outlet side of the aforesaid screen mesh having a breaker perform two functions of the compressing part and the metering and discharging part of a conventional type screw extruder by shearing and compressing and further stirring and plasticizing a fed rubber composition by meshing of gears, and sending the rubber composition to the die head with a predetermined pressure. The gear pump to be used can be a general gear pump for high viscosity; however, it is preferably a gear pump such that a thread groove is provided at a bearing part to allow the scorched rubber generated at an end surface of the gear to be automatically discharged in accordance with the rotation of the shaft of the gear.
When a rubber composition is fed to a screw extruder in the aforesaid composite extruding apparatus of rubber, the rubber composition is sent to the outlet side of the barrel in accordance with the rotation of the screw, and is heated at the same time by friction with the inner surface of the barrel as well as being heated and plasticized by a function of a heating medium flowing through the barrel and the screw shaft. The rubber composition that has reached the ejection outlet of the screw extruder is extruded towards the screen mesh and the breaker plate and receives shear and friction when passing through the screen mesh and the breaker plate, whereby the rubber composition is plasticized and at the same time stirred to fill a space on the feeding side of the gear pump in a state in which the pressure has been reduced by resistance when the rubber composition passes through the screen mesh and the breaker plate. The rubber composition fed to the gear pump receives shear and compression in accordance with the rotation of the gear pump to be further plasticized and stirred, and is sent to the die head with a predetermined pressure and speed to enable precision molding. In other words, in the apparatus of the present invention, plasticization of a vulcanizable rubber composition is carried out by a primary plasticization mainly owing to the function of the barrel and the screw of the screw extruder and the function of the screen mesh and the breaker plate, and a secondary plasticization in the gear pump, whereby the vulcanizable rubber composition is made moldable. Further, the rubber composition is stirred by the screw, the screen mesh, the breaker plate, and the gear pump.
In the composite extruding apparatus of rubber according to the present invention, the height of the flight (flight depth) formed on the aforesaid screw is preferably within a range from (⅕)D to ({fraction (1/15)})D, more preferably within a range from ({fraction (1/7)})D to ({fraction (1/12)})D.
By adopting a screw having a flight height lower by 10 to 20% than the fight height of the conventional screw, the torque applied to the screw can be reduced even in the case of extruding an unvulcanized rubber having a high viscosity, thereby reducing the electric power consumption accompanying the extrusion. Moreover, the screw can be rotated at a high speed and can effectively carry out the plasticization as well as increasing the speed of extrusion.
Further, in the composite extruding apparatus of rubber according to the present invention, the aforesaid screw is preferably provided with a plurality of flight streaks.
By adopting the above-mentioned construction, the stirring efficiency can be improved, the screw can be rotated at a high speed, and the extrusion speed can be increased as compared with a conventionally-used screw with one or two streaks.
The method of extruding an unvulcanized rubber according to the present invention is characterized by use of a composite extruding apparatus of rubber as described herein.
Since the screw length is small, the material residue is reduced, and hence the amount of generated wastes is small. Further, the electric power consumption is small, the throughput is large, and the temperature control is easy, so that extrusion molding of a vulcanizable rubber composition with restrained generation of scorching rubber can be carried out at a low temperature.
In the method of extruding an unvulcanized rubber according to the present invention, the aforesaid screw is preferably rotated at a rotation number within a range from 10 rpm to 120 rpm.
The extruding apparatus used in the method of extruding an unvulcanized rubber according to the present invention has a short screw length and can prevent scorching caused by local heat generation as compared with conventional extruders. As a result, rotation at a high speed increased by 25 to 100% is possible, thereby enabling extrusion molding at a high speed. In the extrusion method of the present invention, the compression ratio is preferably within a range from 1:1.1 to 1:1.3, which is smaller than 1:1.6 of the conventional extruder.